The present invention pertains to a process for the manufacture of a composite material made of polyamide reinforced with long fibers. The process involves the coating of the long fibers with a polyamide prepolymer or oligomer and subsequent heating of the product obtained to achieve polymerization before pultrusion.
The extrusion of a thermoplastic resin, such as a polyamide, in the presence of fibers to improve their mechanical properties is known. Nevertheless, this method is prone to the generation of a material which contains a very high proportion of short fibers, and consequently, does not have the best mechanical properties, particularly excellent flexing characteristics.
The best mechanical properties of plastic resins reinforced with fibers are obtained with resins reinforced with long fibers. To obtain these long-fiber composites, one usually uses the pultrusion technique which involves drawing through a heated die of very long fibers which were first impregnated with a plastic resin. According to this technique, one manufactures rectilinear or curved composite profiles which are highly reinforced in the main direction and whose mechanical characteristics according to this direction can come close to those of some metals, assuming a coherent selection.
A vital and difficult feature of this technique is the impregnation of the fibers. The difficulties would not be so great when the fibers impregnated are thermally hardening resins such as epoxides or polyesters, which at ambient temperature are frequently found in the form of liquids or in solution. The impregnation of the fibers with a thermoplastic resin that is solid at ambient temperature and has a relatively high melting point is more difficult.
The various techniques contemplated so far have been shown to be inefficient or difficult to apply. Such is the case, for example, involving the impregnation of fibers with a thermoplastic polymer powder in a fluidized bed. Another similar case is the passing of the fibers through a thermoplastic polymer solution; the primary shortcoming of this technique is the elimination of great amounts of solvent.
Under these conditions, it is difficult to efficiently reinforce the polyamides with long fibers. These thermoplastic polymers must be kept at high temperatures to be fluid enough to achieve satisfactory impregnation of the fibers, or highly diluted in a solvent, which would involve the problem of eliminating the solvent.